1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) filters, and in particular, to active SAW filters preferably used in radio frequency units and intermediate frequency units of communication equipment.
2. Description of the Related Art
One example of a conventional active filter which compensates for filter insertion loss caused by a resonator is shown in FIG. 5. As seen in FIG. 5, the active filter is a multi-stage filter including a plurality of resonators. In the active filter of FIG. 5, three resonators are connected in a multi-stage arrangement.
Each of the three resonators in the multi-stage filter of FIG. 5 includes an active resonator 31, a coupling device 32 and a .lambda./4-strip line 33 which are connected in multi-stages to similar components of other resonators to form the active filter 30. An internal block diagram of each of the three active resonators 31 is shown in FIG. 6.
Each active resonator 31 requires a dielectric resonator 31a, an amplifier 31b, phase shift lines 31c and coupling devices such as coupling capacitors 31d. The dielectric resonator 31a and the amplifier 31b are connected in the form of a loop via the phase shift lines 31c and the coupling devices 31d to form a positive feedback circuit. The amplifier 31b compensates for insertion loss occurring in the dielectric resonator 31a. Thereby, no-load Q on the active resonator 31 increases, and performance of the active filter 30 is consequently improved.
The use of a plurality of resonators in a multistage active filter requires an amplifier, a phase shifter and a coupling device for each stage of a filter. As a result of the required additional components for each stage of a filter, the size of the filter increases as the number of stages increases. In addition, adjustment of the characteristics of each resonator, and adjustment among the resonators is also required because of the multi-stage arrangement. This causes a problem in which adjustment of the whole filter is very complicated.
The dielectric constant of a dielectric material which can be used for a resonator is approximately 100 at its maximum. In a sub-microwave band (several hundred MHz to 2 GHz) the size of a dielectric resonator is large. For example, let the resonance frequency be 1 GHz. A wavelength .lambda. in a vacuum is approximately 300 mm. Let the dielectric constant be 100. The wavelength shortening rate is 0.1, and wavelength .lambda. is approximately 30 mm. Thus, the length of one .lambda./4-resonator in this case is 7.5 mm. Accordingly, it is impossible to reduce the size of a filter using resonators connected in multiple stages.
In addition, as a result of the active filter having a dielectric resonator 31a, a sufficiently wide pass band cannot be achieved because a dielectric resonator has a relatively narrow pass band. Therefore, a plurality of dielectric resonators 31a must be connected in a multi-stage arrangement to achieve a desired pass band. Although the desired pass band having a sufficient width is obtained by connecting a plurality of dielectric resonators in a multi-stage arrangement, such an active filter suffers from the disadvantages of requiring an increased number of electronic components, an increased size of the active filter, increased difficulty and expense in manufacturing the active filter and increased difficulty in achieving adjustment of the characteristics of each resonator, adjustment among the resonators and adjustment of the overall characteristics of the active filter.